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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
AIDS FOR ANALYTICAL CHEMISTS Colorimetric Determination of Fluoride with an Insoluble Inverse Basic Beryllium-Carboxylate Dye Complex Jothi Ramasamy
*’ and Jack L. Lambert
Department of Chemistry, Kansas State University, Manhattan, Kansas 66506
Colorimetric methods proposed or in use for the determination of fluoride ion have included decolorization of a colored complex such as iron(II1)-thiocyanate ( I ) , displacement of a ligand of one color from its metal chelate of another color as in the zirconium-SPADNS reagent ( 2 ) , displacement of a colored anion from an insoluble salt such as thorium chloranilate ( 3 ) ,and addition of fluoride ion to displace a coordinated water molecule to effect a color change as with the cerium(II1)-alizarin complexone reagent ( 4 ) . The reagent reported here differs from those described above in that an insoluble basic beryllium complex with carboxylate azo dye anion ligands is used as the reagent. Fluoride ion displaces a colored, soluble azo dye anion for spectrophotometric determination. A unique advantage is the lack of interference from sulfate. Normal complexes are those which have a Lewis acid (metal cation) surrounded by coordinated Lewis base ligands. Basic beryllium carboxylate compounds (Figure 1) are “inverse” complexes in which a tetracoordinate oxygen Lewis base is surrounded by four beryllium cations, which are in turn chelated by six bidentate carboxylate anions ( 5 ) . These complexes are insoluble in water if the ligands contain no solubilizing groups other than the chelated carboxylate groups. They exhibit varying degrees of solubility in organic solvents. Water-insoluble complexes of this type will react with fluoride ions to form the more stable Be-F bonds, and release one ligand for each fluoride. The concentration of the released dye ligand, measured spectrophotometrically, is thus directly proportional to the concentration of fluoride ion. The stability constants (6),which are the most important in exchange equilibria, for the formation of the first metalfluoride bond formed
follow the general trend
AI3+ N Sc3+ > Th4+ N Zr4+ > Be2+ > Fe3+> Ce3+ > La3+ The corresponding affinities for sulfate ion, which is the most common and troublesome interference in colorimetric methods for fluoride, follow the trend Fe3+ N Zr4+ > La3+> Ce3+ N Th4+> A13+ > Sc3+>> Be2+ Complexes of beryllium, therefore, afford the opportunity for favorable ligand exchange with fluoride while avoiding interference from sulfate.
EXPERIMENTAL The compounds used were reagent grade or the highest purity commercially available. Aqueous solutions were prepared in deionized water. Present address, Department of Chemistry, North Carolina A&T State University, Greensboro, N.C. 27411. 0003-2700/79/035 1-2044$01 . O O / O
Table I. Data for Different F- Samples F- added,
PPm 0 1
3 5
I 10 15 a
total absorbancea
std. dev.
0.178 0.249 0.403 0.552 0.698 0.941 1.293
0.005 0.004 0.004 0.002 0.003 0.021 0.020
corrected absorbance 0.000
0.071 0.224 0.373 0.519 0.762 1.115
Average of five determinations at 490 nm.
Table 11. Interference Study ion
salt
-
concn, ppm
Blank Ca(NO3)>,4H,O 500 Mg(N03);6H,0 500 KCl 500 NaCl 500 HCl N SO,z- Na,SO, 500 C1NaCl 1000 ~ 0 ~ 3 KH,PO, 10 100 NaNO, NO; S 2 - Na2S.9H,O 100 HCO; NaHCO, 100 OHNaOH low3N CaZ+ Mgz+ K+ Na’ H’
490Aa
490Ab
490AC
0.178 0.552 0.941 0.088 0.426 0.409 0.122 0.362 0.478 0.078 - 0.893 0.094 1.000 0.000 0.523 0.161 0.093 0.646 1.040 0.066 0.553
0.171 0.108 0.310 1.602
a Without fluoride ion. With 5 ppm fluoride ion. With 10 ppm fluoride ion. N o t e : All the absorbance values are the TOTAL absorbance.
Ligand. 4-(2-Hydroxy-l-naphthylazo)-benzoic acid was prepared by coupling diazotized p-aminobenzoic acid with 0naphthol according to the method described by Fierz-David and Blangey for azo dyes (7). Salting out of the azo dye by sodium chloride was omitted. A 0.5% aqueous solution of the dye was prepared. Reagent Papers. Whatman No. 40 filter papers, 7.0-cm diameter, were immersed in the 0.5% azo dye solution for 30 s, the excess dye solution was removed by blotting between filter papers, and the papers were dried at room temperature. The dye-coatedpapers were stored in a brown glass bottle. The reagent papers were prepared immediately before use by immersing in a 0.2% solution of beryllium nitrate, Be(N0J2.3H20,for 60 s. Excess beryllium nitrate solution was removed by washing with several changes of deionized water. After blotting between filter papers, the reagent papers were dried at room temperature and then used. Procedure. The pH of the fluoride sample solution was adjusted to 6.0 with dilute sodium hydroxide or hydrochloric acid. Ten mL of sample was placed in a 100 mm X 15 mm Petrie dish and a freshly-prepared reagent paper added. Gentle agitation and temperature control were provided by a shaking machine that provided temperature control at 25 f 0.5 “C. Swirling also may be done by hand. After 30 min, the solution is filtered and the
c 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
I
R Figure 1. Structure of basic beryllium compound
absorbance measured at 490 nm against a blank prepared in the same manner with deionized water. The value of the blank is subtracted from the absorbances of the samples. Data for the different fluoride samples are shown in Table I. The curve is linear to 15 ppm fluoride. Interferences. Common anions and cations were investigated as interferences (Table 11). Anions were tested for interference as their sodium or potassium salt for positive interference at zero fluoride concentration. Hydroxide ion, because of its close similarity in size and charge density to fluoride, interferes as expected. pH control at 6.0 is essential. Cations which weakly complex or precipitate fluoride, such as calcium and magnesium, can be adequately removed by the cation-exchange resin.
RESULTS AND CONCLUSIONS The 6:4 ratio of dye anions to beryllium cations in the basic complex was confirmed by the method of continuous variations, using M solutions of each component. Some variation in sensitivity with the type and porosity of filter
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paper was observed; Whatman No. 40 paper provided maximum sensitivity. The choice of ligand was severely restricted by several considerations: first, the carboxylate ligand must be highly colored and an azo dye appeared to be the most feasible choice; second, in order to admit six ligands around the {OBe4)6+ nucleus, groups no larger than substituted benzoic acid would appear necessary to avoid steric hindrance; and third, selection of an azo dye of desirable hue and sufficient molar absorptivity would suggest a naphthol group as the coupling component. Further, experience in the preparation of azo dyes suggested that a single pure compound would be obtained if @-naphthol were the coupling component rather than a-naphthol, which often produces a mixture of dyes. The method as described lacks the sensitivity necessary for such purposes as control of fluoridation of public water supplies, but the absence of interference by sulfate makes it potentially useful in other applications.
LITERATURE CITED (1) Snell, F. D.; Snell, C. T. "Colorimetric Methods of Analysis", Vol. 11, 3rd ed.; Van Nostrand Co.: New York, 1949; Chapter 54. (2) Bellack, E.; Schouboe, P. J. Anal. Chem. 1958, 3 0 , 2032. (3) Hensley, A. L.; Barney, J. E.. 11. Anal. Chem. 1960, 32, 828. (4) Belcher, R.;Leonard, M. A,; West, T. S. Talanta, 1959, 2, 92. (5) Cotton, F. A,; Wilkinson, G. "Advanced Inorganic Chemistry", 3rd ed.; John Wiley 8 Sons: New York, 1972; Chapter 7. (6) Sillen, L. G.; Martell, A. E. "Stability Constants", 1964, Special Pub. No. 17; The Chemical Society: London, 1971; Supplement No. 1, Special Pub. No. 25. (7) FierzDavid, H. E.; Bbngey, L. "Fundamental Processes of Dye Chemistry", transl. from the 5th Austrian ed., by P. W. Vittum; Interscience: New York, 1949; p 262.
RECEIVED for review September 29, 1978. Accepted July 9, 1979. This work was supported in part by National Science Foundation Grant CHE-7403024.
Carbide Coating Process for Graphite Tubes in Electrothermal Atomic Absorption Spectrometry Elsa Norval," H. G. C. Human, and L. R. P. Butler National Physical Research Laboratory, CSIR, P.O. Box 395, Pretoria 0007, Republic of South Africa
Automated or semiautomated atomic absorption spectrometers using electrothermal atomization are capable of handling large numbers of samples without the need for continuous operator attendance. The weak link in the automated process has thus far been the graphite tube, whose physical characteristics change on aging ( I , 2). It has also been shown that the type, structure, and reactivity of the graphite influence sensitivities and limits of detection ( 3 ) . The use of tubes coated with pyrolytic graphite solves some problems and in many cases an increase in sensitivity is obtained ( 4 , 5 ) . A major disadvantage of this type of coating is that it is often progressively removed from the surface and recoating of the tubes requires recalibration. Moreover, when graphite surface interference effects are involved, the problem is not always eliminated by such a coating. Bokros (6) has shown that the wear resistance of a pyrolytic carbon-coated article is markedly improved if a thin layer, made up of pyrolytic carbon plus an amount of a carbide additive having good frictional wear characteristics, is created near the outer surface. Such a finding may point to a different type of bonding of the carbon which could also have greater 0003-2700/79/0351-2045$01.00/0
resistance to elevated temperatures and chemical attack. A number of reports have been published which describe treatment of graphite tubes with solutions of metals capable of forming interstitial carbides (7-12). These papers all describe soaking the tube in a solution or applying the solution to the inner tube surface. Both these methods preclude forming a layer of the metal as a first step. This is considered an advantage as any mass of the metal can be sputtered, whereas the concentration of solutions of interstitial carbide-forming metals is generally limited. In addition the use of such solutions results in the metal atoms being interspersed with other substances. A barrier of the protective carbide is therefore not obtained. Furthermore, nowhere is any mention made of the reproducibility of the coating or reinforcing process and information on tube lifetime is scanty. Ortner and Kantuscher (9) stated that the lifetime of tubes treated by their method was similar to that of untreated tubes, Runnels et al. (IO) reported that their coating was stable for the normal useful life of the furnace, and Zatka (12) gave a figure of 400 firings a t temperatures of 2700-2800 O C . This paper describes the application of tungsten and 1979 American Chemical Society
